This invention relates to a photopolymerizable resist material. More particularly, it relates to aqueous processable resist compositions which can be laminated to copper surfaces.
Photopolymerizable resist materials are known from U.S. Pat. No. 3,469,982 which describes preparing a film resist in the form of a photopolymerizable layer sandwiched between a cover sheet and a temporary support. This film resist may then be laminated to copper or copper-laminated board and exposed and processed to provide a photopolymerized layer which acts as a resist for subsequent etching.
Aqueous processable photopolymerizable resists offer significant advantages to the printed circuit industry in terms of lower costs and environmental safeguards. Acid binders have played an important role in the production of such resists. Acid binders are described in German Pat. No. 2,123,702; U.S. Pat. No. 4,273,857; U.S. Pat. No. 3,458,311; and U.K. Pat. No. 1,507,704.
In the manufacture of printed circuit boards from photoresist element, it is necessary to laminate the photosensitive layer of the element to the substrate board. U.S. Pat. No. 3,547,730 discloses the lamination process whereby the layer achieves sufficient adhesion to the substrate to withstand subsequent photoresist processing steps of development, etching and plating.
One of the problems with the use of resists has been adhesion properties. U.S. Pat. No. 3,622,334 described adding hetercyclic nitrogen-containing compounds such as benzimidazole to the photopolymerizable composition to improve the adhesion of photopolymer resist films used in aqueous plating solutions. While this technique is beneficial for a resist which is removed by an organic solvent, it led to stain and scumming on a copper surface to which an aqueous processable, acid binder resist formulation has been laminated, particularly when several days elapsed between lamination and development.